Real preview without rebooting or waiting for over 5 seconds/theme.
Yeah, this is a real problem that I don't think can be solved by wishing it to happen. Not all things are possible with programming, unfortunately; there are real world constraints we have to live with.
AFAIK the current themes require processing since they need to be compiled to CSS that the browser understands, and that requires time. What is sacrificed in time is gained in modularity and composability, which I think is quite a fair tradeoff, unless you plan on changing the CSS constantly (several times a day). This is a scenario I've seen too, but the way to solve it is extremely complex.
But by all means, if you can pull it off, more power to you!